Liquid aeration method and apparatus

ABSTRACT

A method for aerating a liquid such as sewage in a treatment tank, includes arranging a plurality of discrete diffusing surfaces in an optimum pattern about the tank and transmitting an identical quantity of air through each diffusing surface so that a plurality of discrete columns of air bubbles ascend through the liquid in an optimum pattern to the surface thereof. This pattern maximizes the rate of air/liquid interface production by increasing the mean velocity difference between air and liquid, and enhancing the degree of localized microturbulence. The residence time of the air bubbles is also increased. This results in a higher oxygenation efficiency. A diffusing apparatus is also disclosed for facilitating the above process. The apparatus includes a diffusing material having a substantially flat outer surface, and means for connecting the diffusing material in air flow communication with an air conduit disposed adjacent or under the bottom of the tank, such that the outer diffusing surface is substantially horizontal. A plurality of these diffusing apparati may be easily and quickly arranged about the tank in any desired pattern.

States Patent [1 1 Candel 1 Oct. 30, 1973 [76] Inventor: Sebastien M.Candel, 790 Earlhom,

Pasadena, Calif. 91106 [22] Filed: Apr. 21, 1971 [21] Appl. No.: 136,102

[52] US. Cl. 261/122, 261/124 [51] Int. Cl 1301f 3/04 [58] Field ofSearch 261/122, 124; 285/235 [56] References Cited UNITED STATES PATENTS3,048,339 8/1962 Tapleshay 261/122 3,525,436 8/1970 Reckers 1. 261/1243,532,272 10/1970 Branton 261/122 3,396,950 8/1968 Wood 261/1243,317,087 5/1967 Landis 285/235 3,700,266 10/1972 Glehn 285/235 PrimaryExaminer-Tim R. Miles Assistant ExaminerSteven H. MarkowitzAttorney-Christie, Parker & Hale [57] ABSTRACT A method for aerating aliquid such as sewage in a treatment tank, includes arranging aplurality of discrete diffusing surfaces in an optimum pattern about thetank and transmitting an identical quantity of air through eachdiffusing surface so that a plurality of discrete columns of air bubblesascend through the liquid in an optimum pattern to the surface thereof.This pattern maximizes the rate of air/liquid interface production byincreasing the mean velocity difference between air and liquid, andenhancing the degree of localized microturbulence. The residence time ofthe air bubbles is also increased. This results in a higher oxygenationefficiency. A diffusing apparatus is also disclosed for facilitating theabove process. The apparatus includes a diffusing material having asubstantially flat outer surface, and means for connecting the diffusingmaterial in air flow communication with an air conduit disposed adjacentor under thdbottom of the tank, such that the outer diffusing surface issubstantially horizontal. A plurality of these diffusing ap parati maybe easily and quickly arranged about the tank in any desired pattern.

13 Claims, 8 Drawing Figures LIQUID AERATION METHOD AND APPARATUSBACKGROUND OF THE INVENTION This invention relates to the diffusion ofair into a liquid and more particularly to a method and apparatus foraerating sewage during the course of sewage treatment.

A common form of sewage treatment is by the activated sludge process. Inan initial stage of this process, raw sewage influent is mixed withsettled solids or sludge and is aerated for a predetermined period oftime. During this period, the raw sewage and sludge undergo abiochemical process where the solids, organic matter and bacterias areconverted into liquids and activated sludge. More specifically, thesolid matter in the sewage is absorbed and partly oxidized by masses ofbacteria which form into clumps of solid matter, referred to as floc oractivated sludge.

The liquid/floc suspension is then commonly fed to a clarifier where thesludge flocs settle out and are eventuallly partly remixed with theincoming raw sewage. Meanwhile, the liquid remaining in the clarifier isdischarged as purified effluent. Such effluent may be fed through achlorinator for further purification.

Aeration for the activated sludge process is generally provided bybubbling in compressed air through a diffusing device situated in atreatment tank. However, aeration may also occur by mechanicallystirring the water to bring it in contact with ambient air. Thisinvention is concerned with the former method of aeration.

In the past, various methods of aeration by compressed air have beenused. In one such method air is introduced through porous tubes locatedadjacent a side wall of the tank. The rising air bubbles impart anoverall rotary motion to the sewage. This method (socalled spiral flow)has proven least efficient in aerating sewage. The large scale rotarymotion of the sewage swiftly entrains the bubbles to the surface, thusdecreasing the residence time of each air bubble in the liquid.

Most of the energy available from the expanding air bubbles is convertedinto mean flow kinetic energy and further dissipated by the frictionforces at the walls. Most of the energy is thus dissipated far from thediffusion region; the degree of turbulence in this region is smallresulting in a reduced rate of interface production and thus reducedoxygenation efficiency.

The mean velocity difference between the air bubbles and the liquid isalso minimal, further decreasing the rate of interface production.Additionally, a core of poorly aerated sewage develops in the center ofthe tank. As used in this application, the terms rate of interfaceproduction" or the rate of interface renewal are both defined as therate at which the liquid sewage- /air interface is renewed with newmonomolecular layers of low oxygenated sewage.

The porous tubes used in the spiral flow method have a number ofdefects. For instances, only percent of their surface is effective forproducing air bubbles because of the pressure difference between theupper and lower part of the tube. The mechanical mounting of the tubesis expensive and is subject to corrosion by the highly corrosive sewage.Additionally, to avoid clogging the diffusing material, the air must becarefully filtered.

In another method of diffusing air into a liquid, such as sewage, air isdiffused through porous plates built into the base of the tank. To avoidclogging, the air must be carefully filtered. Clogging also occurs whenthe air flow breaks down. After a period of time. the bottom of the tankmust be rebuilt resulting in a high maintenance cost. As the plates haveto resist high mechanical stresses (especially when an air breakdownoccurs) they are relatively thick. This results in higher pressurelosses and thus higher energy comsumption.

At present, all the methods of diffusing air into sewage have one ormore of the following disadvantages: high initial cost, complexconstruction, difficult maintenance, great energy comsumption, lowoxygenation efficiency.

SUMMARY OF THE INVENTION A critical evaluation of existing aerationsystems has uncovered fundamental design misconceptions. A detailedanalysis of the physics and fluid mechanics of the aeration process wasconducted. (It is based on such studies that the spiral flow method wasfound so deficient).

In terms of the physics of diffusion, it was learned that maximumoxygenation efficiency is obtainable only when a large deficiency existsbetween the oxygen concentration in the monomolecular layer of the waterimmediately adjacent an air bubble before it contacts the air bubble toform an interface. At the instant the interface is formed, themonomolecular layer becomes saturated with oxygen. It is necessary toconstantly and as rapidly as possible renew this interface with newmonomolecular layers of low concentration, i.e. the rate of interfaceproduction should be at a maximum.

From a consideration of fluid mechanics, it was discerned that the rateof interface production is maximized when the velocity differentialbetween the air bubbles and the liquid adjacent thereto and beingentrained thereby is maximized. This may result from a higher meanvelocity difference and also a higher degree of microturbulence near theregion of oxygen transfer. The localized turbulence generatessubstantial eddy currents which transport liquid across the streamlinesthereby further increasing the rate of interface renewal.

It was also discerned that the greater oxygenation rate will occur formaximum residence time of the bubbles in the liquid. Thus, the airbubbles should be allowed to rise naturally and not be rapidly entrainedto the surface by an overall liquid flow as in the spiral flow methodabove described. Ideally, no overall liquid motion should exist, butrather the localized turbulence should be maximized especially near thediffusion regions.

The only source of energy in the system considered comes from theisothermal expansion of the gas bubbles as they rise to the surface. Itis this energy which produces the fluid flow in the tank. The characterof this motion is essentially determined by the distribution of thisenergy in the tank. When the gas is introduced in a small region of thetank, i.e. when the energy is released in a small region as in thespiral flow method, the liquid is set in a rapid motion, most of theenergy is dissipated at the walls, and only a small amount is convertedinto turbulent energy in the bubbly region. The rate of interfaceproduction is poor. The mean motion entrains' the bubbles to the surfacethereby reducing their residence time.

If, on the other hand, the available energy is released uniformly in thetank, most of this energy will be converted into turbulent energy. Nolarge scale mean motion will be produced. This results in a higher rateof interface production and a longer residence time.

It was, therefore, concluded from the above study that an efficientmanner of diffusing air into a liquid, such as sewage, is a processwhereby a plurality of discrete diffusing surfaces are arranged at thebottom of the tank in a regular pattern and air is transmitted througheach diffuser to generate a plurality of discrete columns of ascendingair bubbles.

Each air column entrains the adjacent liquid toward the surface. Returnliquid currents flowing back down the tank, set up an area in the liquidsurrounding the rising bubbles of substantially zero velocity. This areais like a cylindrical wall surrounding the column and is hereinafterreferred to as the zero velocity wall.

The difference between the liquid velocity at the zero velocity wall andthe liquid velocity adjacent the bubbles sets up viscous shearingstresses which act to slow down the liquid being entrained by thebubbles, thereby increasing the velocity difference between the airbubbles and the liquid being entrained and thus the rate of interfaceproduction.

A discrete bubble column pattern is necessary to set up descendingcurrents around the columns and to stabilize the general liquidcirculation in the tank. Use of large diffusing surfaces is undesirablesince no control would exist over the liquid flow pattern.

In accordance with the process of this invention, the available energyis spread uniformly in the tank so that no large scale mean motion willbe created and so that the degree of turbulence will be enhanced. Thisalso prevents the formation of a core of poorly aerated liquid in thecenter of the tank.

An optimum distribution of the diffusing surfaces exists and depends onthe geometry of the tank, the system operating parameters, and thesewage quality. The optimum distribution may be determined analyticallyor, more accurately, by a trial and error process of diffuserarrangement and rearrangement in a prototype tank or in the full sizetank. The best distribution maximizes the oxygenation efficiency.

The present invention is also directed to an apparatus for facilitatingthe above process and arrangement. Generally speaking, the apparatuscomprises a diffusing material having a flat outer diffusing surface andmeans for connecting the diffusing material in air flow communicationwith an air inlet conduit situated adjacent or under the tank bottom,such that the outer diffusing surface is substantially horizontal. Aplurality of these diffusing apparati may be arranged in any desiredpattern about the tank.

An important feature of the apparatus of this invention is its easymounting and dismounting on the air supply conduits, so that a trial anderror process may be carried out to achieve maximum oxygenationefficiency. The apparatus is further important in that it is extremelylow in cost and may be discarded when clogged. This feature eliminatesthe need for air filters and the like. It also eliminates air breakdownproblems, i.e. when the air flow drops the liquid may enter the porousmaterial and air supply pipes. Additionally, there is no restartingproblem.

The apparatus is further characterized in having a simple, non-metallicand non-corrosive structure with no moving parts. It is thus quite wellsuited for use in a tank filled with highly corrosive oxygenated sewage.

In another aspect of the apparatus of this invention, the diffusingmaterial is made relatively thin since it is small in outer surface areaso that it is subject to relatively small mechanical stresses. Makingthe diffusing material thin reduces the initial cost thereof, decreasesresulting pressure losses and thereby reduces the overall operatingexpenses and costs.

BRIEF DESCRIPTION OF THE DRAWING These and other aspects and advantagesof this invention are more clearly described with reference to thedrawing wherein:

FIG. 1 is a cross-sectional view of a sewage treatment tank containingan array of diffusing apparati of this invention;

FIG. 2 is a blown-up view of one discrete diffuser showing the meanliquid flow paths and currents the reabout which form the theoreticalbasis of this invention;

FIG. 3 is a side elevation view, partly in section, of a discretediffusing apparatus in accordance with this invention;

FIG. 4 shows how the apparatus in FIG. 3 is connected to an air inletconduit;

FIG. 5 shows an alternative base portion for the apparatus of FIG. 3;

FIG. 6 shows a flow control disc embedded into the body of the apparatusof FIG. 3;

FIG. 7 shows a flow control disc formed in the air inlet conduit shownin FIG. 4; and

FIG. 8 is a sectional view of a segment of an alternative diffusingapparatus of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A sewage treatment tank 10having a flat floor 12, side walls 14 and being of rectangular-shapedcrosssection is shown in FIG. 1 as an example of one type of use for theprocess and apparatus of this invention. Since the precise geometries ofthe tank may vary dependent upon design and economic considerations, itis important that whichever sewage treatment process and apparatus isused, it be made adaptable to any particular tank geometry employed. Asshall be discussed hereinafter, such is the case with the process andapparatus of this invention. Tank 10 is designed and equipped to carryout a basic activated sludge process on liquid sewage contained therein.

An essential aspect of this process is the provision of an efficientaeration apparatus. From what has been discussed above it was discernedthat for the most efficient oxygenation of liquid sewage, a plurality ofdiscrete diffusing surfaces, such as defined on a plurality of diffusers24, should be arranged about the tank bottom to produce an array ofascending columns of air bubbles. Through a trial and error process, tobe described below, an optimum arrangement for any particular tankgeometry is discerned by measuring the oxygenation efficiencydifferential between the air bubbles of each column and the liquid beingentrained toward the surface by the air bubbles.

As shown in FIGS. 1 and 2, diffusers 24 are arranged uniformly about thetank bottom in a series of rows and columns. Each diffuser is connectedto a riser pipe 26 extending vertically upward from an air inletmanifold 28. A source 30 of compressed air is located without the tankand is connected to manifold 28 for supplying an identical quantity ofcompressed air to each of diffusers 24. Each diffuser 24 then generatesan ascending column of isothermally expanding air bubbles 32.

As best shown in FIG. 2, air bubble column 32 rises toward the liquidsurface at a velocity V,,. The rising column sets up a plurality ofascending and descending liquid currents, the ascending liquid currents34 having a velocity V Maximum oxygenation by maximum interfaceproduction requires a maximum velocity differential between V,, and VThis occurs essentially automatically once an optimum arrangement ofdiffusers is discerned and made. More specifically, the descendingliquid currents 36 interact with the adjacent ascending liquid currents34 to set up a zero velocity wall 38 surrounding the bubble column.This, in turn, produces viscous shear stresses which act to slow downthe liquid being entrained upwardly by the air bubbles thereby loweringthe value of V relative to V Oxygenation efficiency is further increasedand maximized, due to an optimum arrangement of diffusers 24, since mostof the energy emitted, as the bubbles rise and expand isothermally, istransferred into localized turbulent energy. Very little energy is lostdue to friction forces at side wall 14 since there is no overall liquidflow. The absence of such overall liquid flow permits the air bubbles torise naturally toward the surface so that their residence time withinthe liquid is maximized. Furthermore, the high degree of localizedturbulence enhances the eddy current flow across the liquid streamlines.These three factors increase the oxygenation efficiency.

An optimum arrangement of diffusers 24 exists for every tank geometry,operational parameters, and liquid influent conditions. An importantaspect of this invention is the ability to readily arrange thediffusers, consistent with the structure of conduit 28, in any desiredpattern to achieve required bubble column patterns. More specifically,and as shall be described in greater detail below, each diffuser isreadily removable from and replaceable on any of risers 26. There aregenerally many more risers defined on manifold 28 than is necessary sothat arrangement and numerous re-arrangement of diffusers 24 may beundertaken to discern, by trial and error, the optimum liquid flowpatterns.

Referring now to a particular diffuser 24, reference is had to FIGS.2-4. Diffuser 24 is of unitary construction which is defined by a body40. Body 40 is made of any cheap resinous material able to resist thesurrounding medium, such as rubber or polyvinylchloride (PVC). Forpurposes of explanation, body 40 is divided into three basic parts: (1)a head 42, (2) a neck 44, and (3) a base 46.

Referring first to base 46 and to FIGS. 3 and 4, it can be seen that thebase is used to fasten diffuser 24 to a selected one of riser pipes 26on conduit 28. As shall be discussed below, base 46 insures that thediffuser is vertically stabilized relative to the tank bottom so thatthe diffusing surface, through which compressed air is passed, issubstantially horizontal.

Base 46 is a hollow circular cylinder of diameter substantially largerthan the diameter of riser pipe 26. The base has opposite ends 48 and 50which are closed by a pair of disc-like membranes 52 and 54. Anothermembrane 56, similar in configuration to membrane 54, is disposed withinbase 46, parallel to membrane 52 and 54, and located intermediatemembranes 52 and 54.

Each of membranes 52, 54 and 56 has a central aperture (58, 60 and 62,respectively) defined therethrough. Apertures 58, 60 and 62 are alignedwith each other and are concentric with the longitudinal axis of thebase. Each membrane is made of the same material as body 40. However,membranes 54 and 56 are made thinner than membrane 52 so that they aremore elastic. In fact, membrane 52 is fabricated so that it isessentially rigid.

Apertures 60 and 62 are designed to receive riser pipe 26 as the base ispushed down thereover. In this regard, apertures 60 and 62 arepreferably of smaller diameter than that of pipe 26. The elastic natureof membranes 54 and 56 permits apertures 62 and 60, respectively, toexpand diametrically to receive riser pipe 26. Additionally, as the baseis pushed down over riser pipe 26, membranes 54 and 56 tend to deformupwardly as their apertures open to receive the pipefThis is importantfor it makes vertical shifting of the diffuser in an upward directiondifficult when the diffuser is mounted on the riser.

The purpose of membrane 52 is two-fold: (1) its aperture 58 provides airflow communication between base 46 and neck 44, and (2) it establishes alimit beyond which base 46 may no longer be pushed down on riser pipe26. In the latter regard, as riser pipe 26 is inserted within base 46through apertures 62 and 60 of membranes 54 and 56 as the base is pusheddown, downward travel of the base will be stopped when the upper end ofthe riser pipe contacts relatively stiff upper membrane 52.

When this downward limit is reached, riser pipe 26 is in air flowcommunication with aperture 58 which has a smaller diameter than that ofriser pipe 26. Membrane 52 is important in that it serves to stabilizediffuser 24 not only vertically, as above described, but angularly aswell. More specifically, the open upper end of riser pipe 26 lies in aplane which, when in contact with the plane of upper membrane 52,prevents tipping of the diffuser relative to the riser pipe. Horizontalmovement of diffuser 24 relative to riser 26 is stayed because of thegripping action of membranes 54 and 56 about the pipe. As shall becomeclearer below, when riser pipe is positioned as shown in FIG. 4, theflat diffusing surface defined on the diffuser is positionedsubstantially horizontal.

Reference is had to head 42 of diffuser body 40 which is now describedwith reference to FIGS. 2 and 3. Basically, head 42 is dish-shaped witha vertical annular outside wall 64 and a flat lower annular floor 66.Floor 66 extends from an inner diameter at the upper extent of neck 44to an outer diameter at side wall 64. The junction of side wall 64 andfloor 66 is smooth.

An annular lip 68 extends inwardly from said wall 64 to define a flatshelf 70 onto which a diffusing material 72 is to rest. The material maythen be glued to head 42 by applying glue 71 about a peripheral edge 73of the material so that the glue also contacts the top of side wall 64.

Referring now more specifically to material 72, it is preferably porousin nature and is defined as a circular disc of predetermined thickness.It may be made of any porous material, such as ceramics, for example.Material 72 has an upper flat diffusing surface 74 and a lower flatdiffusing surface 76 parallel with surface 74. The outer periphery oflower surface 76 is designed to rest on annular shelf 70. As will bedescribed below with reference to FIG. 8, use of a cellular diffusingmaterial is also comtemplated in this invention.

Material 72 is made as thin as possible and yet still withstand themechanical stresses built up in a liquid filled tank. Thus cuts down theinitial cost of the diffuser. Additionally, having a relatively thindiffusing material cuts down the air pressure losses therethrough andthus the energy consumption. Such thin diffusing material is thus muchmore efficient in oxygenating the liquid than those prior art methodsusing thick porous ceramic plates at the tank bottom.

An air distribution space 78 is defined directly beneath material 72 andis peripheral bounded by lip 68. The lower part of space 78 communicateswith neck 44 so that air transmitted through the neck may be uniformlydistributed about lower surface 76 of material 72. An even and uniformlyarranged column of air bubbles will thereby be generated from diffusingsurface 74 as air passes through the pores of diffusing material 72.

Reference is now had to neck 44. Functionally speaking, neck 44 is ashock absorbing and vibration damping member. The walls 80 of neck 44are defined as part of the unitary body 40 which is made of rubber orPVC. Walls 80 are somewhat thinner, so as to provide for a slightlygreater elasticity, and are shaped like two truncated cones joined attheir smaller bases. A large base 82 of one cone is connected to uppermembrane 52 of base 46; whereas a large base 84 of the other conesupports head 42. Base 84 is of substantially greater diameter than base82.

The slight elastic nature of neck 44 permits temporary and limitedhorizontal and vertical deformations thereof so as to absorb shocks andto damp vibrations caused by air diffusing through material 72.

In operation, and with reference to FIGS. 1-4, a plurality of diffusers24 are assembled for use in tank 10. An initial arrangement of thesediffusers along the tank bottom is made by mounting various ones of thediffusers on certain riser pipes 26. Each diffuser is mounted bygrasping base 46 and pushing down on it until the riser pipe travelsthrough apertures 60 and 62 of membrane 56 and 54 and eventually buttsup against mem brane 52. The diffuser is then properly vertically andhorizontally positioned and stabilized so that diffusing surface 74 ofmaterial 72 is disposed substantially parallel to the liquid surface.

When the initial arrangement is made, compressed air is fed from thesource through manifold 28 to each of the riser pipes. Those riser pipesnot connected to a diffuser may be plugged so as not to interfere withthe air bubble and liquid flow pattern generated in the tank.

With regard to a specific diffuser 24 connected to a specific riser pipe26, air is transmitted through the pipe and into the interior of neck 44through aperture 58 in membrane 52. The air then proceeds intodistribution space 78 where it is uniformly distributed about lowersurface 76 of material 72. The air then passes through the pores inmaterial 72 and then out into the liquid through diffusing surface 74.Since surface 74 is flat and parallel to the tank bottom, a uniformascending column of air bubbles 32 is generated.

From a study of the liquid flow and air bubble patterns generated, itmight be desirable to change the arrangement of diffusers and/or addmore or subtract some. It is thus an important aspect of this inventionwhich enables a person to do this quickly and efficiently. Each diffuseris readily removable from a riser by pulling up on base 46. It may thenbe replaced on another riser or not used at all.

Eventually, an optimum arrangement of diffusers 24 about tank bottom 12is made. This arrangement will produce a high rate of interface renewal.In the optimum arrangement, most of the energy generated, as the airbubbles of each column rise and expand isothermally, is transferred tolocalized turbulence and is not lost by friction forces at side walls14. Thus, the bubble residence time is maximized as are the eddycurrents flowing across the liquid streamlines. All of these factorscontribute to a high rate of oxygenation.

An important aspect of this invention is the ability to discard anydiffuser that has become clogged and to replace it with a new one. Theextremely low cost of the diffuser permits such discarding. Since thediffuser may be thrown away, the quality of air flow is not important.Therefore, no costly air filtration devices are needed.

Another advantage of this invention is that there is no air breakdownproblem. More specifically, liquid entering diffuser 24 through thepores in material 72 due to a drop in air pressure may be easily forcedout when the air pressure is brought back to normal. There is norestarting problem when a thin porous diffusing surface is used.

An alternative base portion of a diffuser of this invention is shown inFIG. 5 and is designated generally by the reference numeral 86. Base 86is similar to base 46 in that it forms part of an overall diffuser bodywhich is fabricated of rubber or PVC. The interior of base 86 is in theform of a circular cylindrical bore 88 having an open upper end 90defined at the lower extremity of the interior of neck 44 and an openlower end 92. An annular notch 93 is cut into a side wall 94 of base 86beneath lower end 92. The notch extends downwardly to an open lower end96 of base 86. An annular shelf 98 is thus defined in side wall 94 justbeneath bore 88.

A flat, rigid, disc-like membrane 100, which is preferably fabricated ofmetal or plastic, is disposed against shelf 98 and is affixed theretosuch as by glueing or the like. Membrane 100 has a central aperture 102of lesser diameter than that of bore 88. The purpose of membrane 100 isto control the flow of air through the diffuser.

Base 86 differs from base 46 of diffuser 24 in that it is designed to bepushed into a hole 104 in manifold 28, which is shown in FIG. 4 asaccommodating a riser pipe 26. When base 86 is used on a diffuser, ariser pipe is not needed. The base is essentially a plug therebyenabling the diffuser to be readily plugged in and out of manifold 28through hole 104. The lower limit of travel of base 86 into hole 104 isdefined when an annular flange 105 of the base contacts an outer surfaceof the manifold.

Flow control ring 100 may also be employed in the system of FIGS. 1-4wherein a riser pipe is used to connect the diffuser to manifold 28.More specifically, ring 100 may be disposed within membrane 52 of base46 in the manner shown in FIG. 6; or it might be disposed at the upperend of riser pipe 26 by being fixed to an annular shelf 107 notched intoa side wall 106 of the pipe at such upper end (see FIG. 7). Wherever itis used, ring 100 controls the air flow through its constricted opening102.

As indicated above, cellular diffusion materials are within thecontemplation of this invention. A diffuser using a cellular diffusingmaterial would have a base 46 and neck 44 identical to that of diffuser24. The head, however, would differ. More specifically, and withreference to FIG. 8, a cellular diffusion material 108, such aspolyurethane with open cells, is shown. For a good diffusion, thepolyurethane foam has to be much thicker than porous ceramic plates,such as material 72. Additionally, since the adequate foam permitshorizontal movement of air, a distribution space, such as space 78, neednot be designed into the head.

With the above in mind, a head 110, for carrying a polyurethanediffusing material, has the shape of a reverse dome. Cellularpolyurethane 108 completely fills the interior space of the dome and hasa flat circular outer diffusing surface 109 which, when the diffuser ismounted to manifold 28, is parallel to the floor of tank 18. Material108 may be affixed to the dome by being glued about its upper peripheryto the upper edge of the dome side wall in a manner similar to theaffixation of material 72 to side wall 64 of head 42. Glueing can beeliminated by fabricating the upper edge of the dome side wall with aninwardly extending annular flange (not shown) to serve as a lid on thefoam.

In operation, air enters head 1 through the interior of a neck 44' andis automatically uniformly distributed through the polyurethane foam,since the latter is cellular. The air is then passed through the foamand is generated out from surface 109 into the adjacent liquid as anascending air bubble column of the type required by this invention formaximum interface production.

What has been described, therefore, is a method and apparatus forincreasing oxygenation in a sewage treatment tank by providing a highrate of interface production. Among the factors contributing to suchhigh rate are: (1) maximum velocity difference between the rising airbubbles and the liquid being entrained thereby, (2) natural flow of airbubbles to the liquid surface, and (3) large number of eddy currentsacross the liquid streamlines occasioned by localized turbulence. Thefactors are realized in the method and apparatus of this invention.

Although the present invention has been described with regard to thespecific embodiments disclosed, it is obvious that numerousmodifications, additions and changes may be made thereto withoutdeparting from the spirit of the invention as defined in the followingclaims.

I claim:

1. Apparatus for diffusing air into a liquid through a conduit, theconduit having receiving means adapted to receive the apparatus in airflow communication, the apparatus comprising:

a. a diffusing material having a substantially flat outer diffusingsurface; and

b. means for connecting the diffusing material in air flow communicationwith the receiving means of the conduit such that the outer diffusingsurface is substantially horizontal, the connecting means including:

i. means supporting the diffusing material;

ii. means for securing the apparatus directly to the conduit receivingmeans and including an air flow passage therethrough for communicatingair from the conduit through the passage; and

iii. means joining the supporting means to the securing means forabsorbing shocks and damping vibrations felt by the diffusing material,the interior of the joining means defining an air flow passage forestablishing air communication between the air flow passage of thesecuring means and the diffusing material, said joining means includinga first truncated conical section and a second truncated conicalsection, the two sections being joined at their smaller diameter ends toform a reduced diameter neck between the diffusing material and thesecuring means, the walls of said first and second section being of thinrelatively flexible material to permit the diffusing material to tiltand move relative to the securing means.

2. The apparatus of claim ll, wherein the connecting means is fabricatedof a non-corrosive material.

3. The apparatus of claim 1, further comprising an air distributionchamber communicating between an inner surface of the diffusing materialand the interior of the joining means, the chamber acting to distributeair uniformly about the inner surface of the diffusing material so thatit is diffused uniformly therethrough.

4. The apparatus of claim 1, wherein the diffusing material is porous innature.

5. The apparatus of claim 1, wherein the diffusing material is cellularin nature.

6. The apparatus of claim 1, wherein the connecting means is fabricatedof a resinous material.

7. The diffuser of claim I, wherein the receiving means of the conduitincludes an aperture defined through the outer surface of the conduitand the securing means comprises:

a. an elongate plug having an outer housing in which is disposed aninner chamber communicating between a pair of open opposite ends, thesecuring means air flow passage being defined through the chamberbetween the open opposite ends; and

b. flow control means disposed within the chamber adjacent one endthereof and closing such end but for an opening disposed therethrough,the opening having an effective area less than the crosssectional areaof the chamber.

8. The diffuser of claim 7, wherein the elongate plug housing has anoutwardly extending annular flange adjacent the other end of thechamber.

9. The diffuser of claim 8, wherein the elongate plug housing is taperedfrom a maximum cross-sectional area adjacent the annular flange, to aminimum crosssectional area adjacent the one end of the chamber.

10. Apparatus for diffusing air into a liquid through a conduit, theconduit having receiving means adapted to receive the apparatus in airflow communication, the apparatus comprising:

. a. a diffusing material having a substantially flat outer diffusingsurface; and

b. means for connecting the diffusing material in air flow communicationwith the receiving means of the conduit such that the outer diffusingsurface is substantially horizontal, the connecting means including:

i. means supporting the diffusing material; ii. means for securing theapparatus directly to the conduit receiving means and including an airflow passage therethrough for communicating air from the conduit throughthe passage; and iii. means joining the supporting means to the securingmeans for absorbing shocks and damping vibrations felt by the diffusingmaterial, the interior of the joining means defining an air flow passagefor establishing air communication between the air flow passage of thesecuring means and the diffusing material.

c. the receiving means of the conduit including a generally verticallyextending riser pipe and the securing means including:

i. an elongate housing having an inner chamber therein, the chamberhaving an expandable air inlet opening at one end and a fixed air outletopening at another end, the securing means air flow passage beingdefined through the chamber between the air inlet and outlet openings;

ii. an elastic membrane disposed across the chamber at the one end andhaving a central aperture therethrough defining the expandable air inletopening; and

iii. a substantially rigid membrane disposed across the chamber at theother end and having a central aperture-therethrough defining the airoutlet opening with an effective area greater than the minimum effectivearea of the air inlet opening.

11. The apparatus of claim 10, wherein the securing means furthercomprises an additional elastic membrane disposed across the chamberintermediate the ends thereof and having a central aperture therethroughwith a minimum effective area less than the mean cross-sectional area ofthe riser pipe.

12. The diffuser of claim 10, wherein the substantially rigid membraneincludes flow control means defined therewithin for constricting thearea of the central opening through the rigid membrane to control theflow of air therethrough.

13. The diffuser of claim 10, wherein the riser pipe has an internalchamber communicating between an open lower end at the conduit and anopen upper end, and flow control means defined in the internal chamberadjacent the open upper end and closing this end but for an openingdefined through the flow control means, the opening having an effectivearea less than the crosssectional area of the internal chamber.

1. Apparatus for diffusing air into a liquid through a conduit, theconduit having receiving means adapted to receive the apparatus in airflow communication, the apparatus comprising: a. a diffusing mateRialhaving a substantially flat outer diffusing surface; and b. means forconnecting the diffusing material in air flow communication with thereceiving means of the conduit such that the outer diffusing surface issubstantially horizontal, the connecting means including: i. meanssupporting the diffusing material; ii. means for securing the apparatusdirectly to the conduit receiving means and including an air flowpassage therethrough for communicating air from the conduit through thepassage; and iii. means joining the supporting means to the securingmeans for absorbing shocks and damping vibrations felt by the diffusingmaterial, the interior of the joining means defining an air flow passagefor establishing air communication between the air flow passage of thesecuring means and the diffusing material, said joining means includinga first truncated conical section and a second truncated conicalsection, the two sections being joined at their smaller diameter ends toform a reduced diameter neck between the diffusing material and thesecuring means, the walls of said first and second section being of thinrelatively flexible material to permit the diffusing material to tiltand move relative to the securing means.
 2. The apparatus of claim 1,wherein the connecting means is fabricated of a non-corrosive material.3. The apparatus of claim 1, further comprising an air distributionchamber communicating between an inner surface of the diffusing materialand the interior of the joining means, the chamber acting to distributeair uniformly about the inner surface of the diffusing material so thatit is diffused uniformly therethrough.
 4. The apparatus of claim 1,wherein the diffusing material is porous in nature.
 5. The apparatus ofclaim 1, wherein the diffusing material is cellular in nature.
 6. Theapparatus of claim 1, wherein the connecting means is fabricated of aresinous material.
 7. The diffuser of claim 1, wherein the receivingmeans of the conduit includes an aperture defined through the outersurface of the conduit and the securing means comprises: a. an elongateplug having an outer housing in which is disposed an inner chambercommunicating between a pair of open opposite ends, the securing meansair flow passage being defined through the chamber between the openopposite ends; and b. flow control means disposed within the chamberadjacent one end thereof and closing such end but for an openingdisposed therethrough, the opening having an effective area less thanthe cross-sectional area of the chamber.
 8. The diffuser of claim 7,wherein the elongate plug housing has an outwardly extending annularflange adjacent the other end of the chamber.
 9. The diffuser of claim8, wherein the elongate plug housing is tapered from a maximumcross-sectional area adjacent the annular flange, to a minimumcross-sectional area adjacent the one end of the chamber.
 10. Apparatusfor diffusing air into a liquid through a conduit, the conduit havingreceiving means adapted to receive the apparatus in air flowcommunication, the apparatus comprising: a. a diffusing material havinga substantially flat outer diffusing surface; and b. means forconnecting the diffusing material in air flow communication with thereceiving means of the conduit such that the outer diffusing surface issubstantially horizontal, the connecting means including: i. meanssupporting the diffusing material; ii. means for securing the apparatusdirectly to the conduit receiving means and including an air flowpassage therethrough for communicating air from the conduit through thepassage; and iii. means joining the supporting means to the securingmeans for absorbing shocks and damping vibrations felt by the diffusingmaterial, the interior of the joining means defining an air flow passagefor establishing air communication between the air flow passage of thesecuring means and the diffusing material. c. the receIving means of theconduit including a generally vertically extending riser pipe and thesecuring means including: i. an elongate housing having an inner chambertherein, the chamber having an expandable air inlet opening at one endand a fixed air outlet opening at another end, the securing means airflow passage being defined through the chamber between the air inlet andoutlet openings; ii. an elastic membrane disposed across the chamber atthe one end and having a central aperture therethrough defining theexpandable air inlet opening; and iii. a substantially rigid membranedisposed across the chamber at the other end and having a centralaperture therethrough defining the air outlet opening with an effectivearea greater than the minimum effective area of the air inlet opening.11. The apparatus of claim 10, wherein the securing means furthercomprises an additional elastic membrane disposed across the chamberintermediate the ends thereof and having a central aperture therethroughwith a minimum effective area less than the mean cross-sectional area ofthe riser pipe.
 12. The diffuser of claim 10, wherein the substantiallyrigid membrane includes flow control means defined therewithin forconstricting the area of the central opening through the rigid membraneto control the flow of air therethrough.
 13. The diffuser of claim 10,wherein the riser pipe has an internal chamber communicating between anopen lower end at the conduit and an open upper end, and flow controlmeans defined in the internal chamber adjacent the open upper end andclosing this end but for an opening defined through the flow controlmeans, the opening having an effective area less than thecross-sectional area of the internal chamber.